Cytoskeleton System
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Transcript of Cytoskeleton System
CytoskeletonCytoskeleton SystemSystem
CytoskeletonCytoskeleton SystemSystem
XiamixinuerXiamixinuer··Yilike Yilike
Chapter 8
Teaching Requirements:
1. Mastering: concepts of the cytoskeleton; structure, chemical composition, and assembly of microtubules and microfilaments.
2. Comprehending: functions of microtubules and microfilaments.
3. Understanding: functions of the cytoskeleton; types and functions of intermediate filaments.
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The cytoskeleton
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1. IntroductionA. Conception of Cytoskeleton (Narrow sense)A complex network of interconnected microfilaments, microtubules and intermediate filaments that extends throughout the cytosol.
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The comparison among three types of the cytoskeleton
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B. Techniques for studying the cytoskeleton
Fluorescent microscopy and Electron
microscopy :
Immunofluorescence: fluorescently-labeled antibody
Fluorescence: microinject into living cells
Video microscopy: in vitro motility assays
Electron: Triton X-100, Metal replica
Drugs and mutations (about functions)
Biochemical analysis(in vitro)
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Fluorescence microscopy
actin
microtubules
filamin
microfilaments
cytoskeleton
microtubules
microtubules
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C. The self-assembly and dynamic structure of cytoskeletal filaments
Each type of cytoskeletal filament is constructed
from smaller protein subunits.
The cytoskeleton is a network of three filamentous
structures.
The cytoskeleton is a dynamic structure with many
roles.
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D. The function of the cytoskeleton
Structural support
Internal framework maintaining position of the organelles
Machinery required for movement of materials and organelles within cells
Force generating elements responsible for movement of cells from one place to another
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2.Microtubule, MTA. Structures:
Hollow
Tubular structures 25nm in diameter
Assembled from protein tubulin
The tubulin consists of alpha-beta tubulin
heterodimers arranged in rows (protofilaments)
Form cytoskeleton, mitotic spindle, centrioles, core
of cilia and flagella
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a and ßTubulin heterodimers are the protein building blocks of MTs
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Arrangement of protofilaments in singlet, doublet, and triplet MTs
Singlet Doublet Triplet
A
B
A
B
CIn cilia and flagella
In centrioles and basal bodies
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Assembling process of MT
+
-
PE
DA
L
OUTSIDE OF THE BODY
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tubulin tubulin
heterodimerassemble Head tail connection
profilament
MT( 13)
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CROSSSECTION
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B. MTs assemble from microtubule-organizing centers
(MTOCs)
Microtubule-organizing centers (MTOCs):is the region to assemble MT,Where
includes-tubulin.MTOCs:include Centrosome, Mitotic spindle
and Basal body.
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Microtubule-organizing centers (MTOCs)
(1) Interphase: Centrosome
Dynamic instability
(2) Dividing cell:
Mitotic spindle
Dynamic instability
(3) Ciliated cell: Basal body
Stability
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Centrosome is a microtubule organizing center, MTOCs
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Centrosome containing a pair of centrioles
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Centrioles
Centrioles are short cylinders with a 9 + 0 pattern of microtubule triplets.
Centrioles may be involved in microtubule formation and disassembly during cell division and in the organization of cilia and flagella.
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MT are nucleated by a protein complex containing -tubulin
The centrosome is the major MTOC of animal cells
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Experiments supporting that centrosome is the MTOC
Treat cell with colcemid
Cytosolic MTs depoly, except those in centrosome
Remove colcemid
Tublin repoly
Expla I: MTOC nucleate poly of tubulins
Expla II: MTOC gather MTs in cytosol
centrosome + Tubulins MT
+ Tubulins No
A
B
Why the centrosome can act as MTOC?
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Cilia and flagella
Cilia (small and numerous) and flagella (large and single) have a 9 + 2 pattern of microtubules and are involved in cell movement.
Cilia and flagella move when the microtubule doublets slide past one another.
Each cilium and flagellum has a basal body at its base.
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Basal body structure
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C. Characteristics of MT assembly
Dynamic Dynamic instability due to instability due to the structural the structural differences differences between a growing between a growing and a shrinking and a shrinking microtubule end.microtubule end.
GTP cap;GTP cap;
Catastrophe: Catastrophe: accidental loss of accidental loss of GTP cap;GTP cap;
Rescue: regain Rescue: regain of GTP capof GTP cap
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Microtubules have a Microtubules have a
plus and minus ends.plus and minus ends.
Typically the minus is Typically the minus is
for anchoring and the for anchoring and the
plus is for growing.plus is for growing.
The transition The transition
between MT growth between MT growth
and MT shrinking is and MT shrinking is
controlled in cells by controlled in cells by
special proteins..special proteins..
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Drugs affect the assembly of MTs
(1) Colchicine
Binding to tubulin dimers, prevent MTs polymerization
(2) Taxol
Binding to MTs, stabilize MTs
These compounds are called antimitotic drugs, and have application in medical practice as anticancer drugs
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D. Microtuble-associated proteins (MAPs)
MAPs modulate MT structure, assembly, and function
Katanin like proteins MAPs
Tau: In axon, cause MTs to form tight bundles
MAP2: In dendrites, cause MTs to form looser bundles
MAP1B: In both axons and dendrites to form crossbridge
between microtubules
Control organization
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MAP2 associated with brain MTs
5. Functions of MTs• A. Maintenance of cell shape(constitute the
centriols and cilia or flagella).• B. Cell motility (see in cilia or flagella).• C. Chromosome movements in cell division • D. Organelle movement (MT associated motor
proteins: kinesins: towards + end (anterograde transport) Golgi to ER or PM traffic;dyneins: towards - end (retrograde transport) ER to Golgi traffic.)
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5. Functions of MTs
A. Maintenance of cell shape(constitute the centriols and cilia or flagella).
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No centrioles in Plant and fungi
A pair of centrioles are surrounded by electron dense pericentriolar material.
Centrioles contain nine evenly spaced fibrils, each containing three microtubules, A, B and C tubules.
A tubule is connected to the center of the centriole by a radial spoke.
Centrioles are in pairs and at right angles to each other.
Structure
Constitute the centriols and cilia or flagella
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Constitute the centriols and cilia or flagella
5. Functions of MTs• B. Cell motility (see in cilia or
flagella).
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A comparison of the beating of flagella and cilia
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Ultrastructure of a eukaryotic flagellum or cilium
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SPERM MOVEMENT
CILIA MOVEMENT
Motility of M
T(
CIL
IA,F
ILA
GE
LA
MO
VE
ME
N
T
)
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Motor molecules and the cytoskeleton
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Motor molecules and the organelle
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Dyenin arms responsible for sliding
Crosslinks and spokes responsible for bending
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B. Transport in the cytoplasm
MT associated motor proteins:
kinesins: towards + end (anterograde transport) Golgi to ER or PM traffic
dyneins: towards - end (retrograde transport) ER to Golgi traffic
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C. Movement of chromosomes
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3. Microfilament, MFA. MFs are made of actin and involved in cell motility.
Using ATP, G-actin polymerizes to form MF(F-actin)
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G-actin
F-actin
PLUS END
MINUS END
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G-actin Dimer Trimer
F-actin+end
-endAssembly of MF
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B. MF assembly and disassembly
Characteristics:
(1) Within a MF, all the actin monomers are oriented in the same direction, so MF has a polarity
Myosin is molecular motor for actins.
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(2) In vitro, (Polymerization) both ends of the MF grow, but the plus end faster than the minus.
Because actin monomers tend to add to a filament’s plus end and leave from its minus end---- “Tread-milling”
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(3) Dynamic equilibrium between the G-actin and polymeric forms, which is regulated by ATP hydrolysis and G-actin concentration.
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(4) Dynamic equilibrium is required for the cell functions. Some MFs are temporary and others permanent.
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C. Specific drugs affect polymer dynamics
Cytochalasins:
Prevent the addition of new monomers to existing MFs, which eventually depolymerize.
Phalloidin:
A cyclic peptide from the death cap fungus, blocks the depolymerization of MF
Those drugs disrupt the monomer-polymer equilibrium, so are poisonous to cells
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D. Actin-binding proteins
The structures and functions of cytoskeleton are mainly controlled by its binding proteins
(1) Monomer-sequestering proteins
Bind with actin monomers and prevent them from polymerizing.
thymosin and ( profilin) Promoting the assembly of MF
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(2) MF-binding proteins
Functions of MF• (1) Maintenance of cell shape and enforce
PM to change cell shape( i.e.Microvillus: Support the projecting membrane of intestinal epithelial cells)
• (2) Cell migration or motility (as in pseudopodia)
• (3) To form contractile ring in cell division: At cytokinesis
• (4) Muscle contraction : Sarcomere is the unit of the muscle cells.
• (5)Cytoplasm streaming54
Functions of MF• (1) Maintenance of cell shape
and enforce PM to change cell shape( i.e.Microvillus: Support the projecting membrane of intestinal epithelial cells)
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Microvillus: Support the projecting membrane of intestinal epithelial cells
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A structure role of microfilaments
Microfilaments (Actin filaments)
Microvilli
Intermediate filaments
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E. Functions of MFs
(2) Cell migration (Fibroblast et al)
Platelet activation is a controlled sequence of actin filament severing,uncapping, elongation,recapping, and cross-linking.
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(3) To form contractile ring in cell division :
At cytokinesis
E. Functions of MFs
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E. Functions of MFs
(4) Muscle contraction
Organization of skeletal muscle tissue
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Sarcomere
5)Cytoplasm streaming:The streaming of cytoplasm in a circular motion around the cell observed in some plants, particularly young sieve tube elements.
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Proteins play important roles in muscle contraction
Myosin: The actin motor protein
ATPase
Binding sites
Myosin II--Dimer
Mainly in muscle cells
Thick filamemts
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Tropomyosin, Tm and Tropnin, Tn
Ropelike molecule
Regulate MF to bind to the head of myosin
Complex, Ca2+-subunit
Control the position of Tm on the surface of MF
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Excitation-contraction
coupling process
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Intermediate filaments, IFs
IFs are the most abundant and stable components of the cytoskeleton
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Class I. + II. (MW 40 - 70 000) Cytokeratinsepithelial cells ( > 20 isoforms, skin, hair, nails)
Class III. (MW ~53 000) Vimentin cells of mesenchymal originDesmin muscle GFAPs astroglial cells (= Glial fibrillary acidic proteins)
Class IV. (MW 130 , 100 and 60 000)Neurofilament proteins in neural cells
Class V. (MW = 65-75 000)Nuclear lamins inside surface of the inner nuclear membranemost dynamic
INTERMEDIATE FILAMENT PROTEIN MONOMERS: (cell-type-specific)
(~ 310 amino acids)
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monomer
coiled-coil dimer
staggeredanti-parallel tetramer
twotetramers
helical arrayoftetramersmade of8 protofilaments
Functions of IF1.Maintenance of cell shape,2.Anchore of nucleus and certain
other organelles,3.Formation of nuclear lamina
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The comparison among three types of the cytoskeleton
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Summary of cytoskeletonSummary of cytoskeleton
• Three types of cytoskeletal filaments are common to many Three types of cytoskeletal filaments are common to many eucaryotic cells and are fundamental to the spatial eucaryotic cells and are fundamental to the spatial organization of these cells.organization of these cells.
• The set of accessory proteins is essential for the controlled The set of accessory proteins is essential for the controlled assembly of the cytoskeletal filaments(includes the motor assembly of the cytoskeletal filaments(includes the motor proteins: myosins, dynein and kinesin)proteins: myosins, dynein and kinesin)
• Cytoskeletal systems are dynamic and adaptable.Cytoskeletal systems are dynamic and adaptable.
• Nucleation is rate-limiting step in the formation of a Nucleation is rate-limiting step in the formation of a cytoskeletal polymer.cytoskeletal polymer.
• Regulation of the dynamic behavior and assembly of the Regulation of the dynamic behavior and assembly of the cytoskeletal filaments allows eucaryotic cells to build an cytoskeletal filaments allows eucaryotic cells to build an enormous range of structures from the three basic filaments enormous range of structures from the three basic filaments systems. systems.
Homework for cytoskeleton system
1. Conception types and the functions of the cytoskeleton
2. Structures of MT
3. building blocks of MTs and MFs
4. Arrangement of protofilaments
5. MTOC and its elements
6. Specific drugs stabilize MTs or MF
7. Functions of MTs and MFs
8. Cytoskeletal systems are dynamic and adaptable.
Nucleation is rate-limiting step in the formation of a
cytoskeletal polymer.Regulation of the dynamic behavior
and assembly of the cytoskeletal filaments allows eucaryotic
cells to build an enormous range of structures from the
three basic filaments systems. 72